This invention relates to fiber reinforced composite materials, and in particular to pultruded articles such as fuse tubes.
There are various applications for hollow elongated members having high burst strengths, such as the tubular casings for electric current limiting fuses. Current limiting fuses generally include a fusible element which is severed when the electrical current in an electrical power line being monitored exceeds a predetermined limit, and a filler for quenching the arc created when current servers the fusible element. Since the foregoing arc releases substantial heat to, in turn, generate high gas pressures and thermal shock, a casing or tube must be provided around the fuse to contain the explosive forces released and to prevent arcing to ground when the fuse is blown. Such fuse casing should have high burst strength and a high resistance to heat shock. Furthermore, such casings should be capable of manufacture at high production rates and low cost. Compactness is a desired feature, with the casing having thin walls and light weight, but the casing must still perform its intended functions. The fuse casing must be an electrical insulator and resistant to thermal shock. Fuse casings should have dimensional stability in longitudinal, radial, and peripheral (i.e. circular for tubular shapes) directions.
Virtually all fuse tubes in current use are composed of vulcanized fiber, composite materials and ceramics. The composite materials are reinforced with multiple layers of fiber glass --some are composed of only woven fiber glass fabric, and others are combinations of various types of fiber glass reinforcement. Fiber glass conducts electricity poorly. Fiber glass is particularly suitable for fabricating elongated pieces of uniform cross section by the manufacturing process of pultrusion ("uniform cross section" means that that cross section is constant along the length of the piece; many pultruded products have non-uniform regions at a given cross section). Glass fibers can be arranged to provide high strength. Fiber glass products are often flexible, facilitating the assembly of end caps used in fuse tubes. Fuse tubes composed of glass fibers or rovings wound about the inner components of the fuse are disclosed in U.S. Pat. Nos. 2,929,900 (White 1960) and 3,846,727 (Harmon 1974). U.S. Pat. No. 2,727,961 (Smith 1955) discloses a fuse tube having an inner liner for generating arc extinguishing gases, and an outer tubular member wound about the liner and composed of a woven fiber glass cloth or fabric which has been impregnated with resin to bond the glass fibers together. U.S. Pat. No. 3,911,385 (Blewitt et al. 1975) discloses a fuse casing composed of a glass fiber cloth which is impregnated with a melamine resin and has an expoxy resin coating. U.S. Pat. No. 3,979,709 (Healy 1975) discloses a pultruded composite fuse tube construction having an inner layer of woven glass fabric, an intermediate layer of glass fiber mat having non-woven, randomly oriented fibers, and an outer layer of woven glass fiber fabric. The latter patent further discloses the incorporation of multiple layers of mat and a layer of fiber glass rovings between the mats. In U.S. Pat. No. 3,984,800 (Salzer et al. 1976), another pultruded fuse casing is described which includes an inner layer of glass fiber rovings, an intermediate layer of non-woven glass fiber mat and an outer layer of woven glass fiber fabric. In such composite fuse tube constructions, fiber glass fabric is employed because the axial and circumferential fibers combine to provide high burst strength, the fiber glass mat is employed to provide additional strength at a lower cost than the fabric, and rovings are incorporated to facilitate the pulling of the product through pultrusion dies during the manufacturing process.
U.S. Pat. No. 3,986,157 (Salzer et al 1976) discloses a prismatic fuse casing (the term "tube" as used herein means any hollow, elongated member including those of prismatic configuration; however, U.S. Pat. No. 3,986,157 specifically discloses a prismatic member) having an outer layer of woven glass fiber fabric, an intermediate layer of non-woven glass fiber mat, an inner layer of fabric, and four bundles of glass fibers at the respective corners of the member. U.S. Pat. No. 4,124,836 (Wilks 1978) discloses a composite fuse having layers of non-woven fiber glass mat sandwiched between layers of woven fiber glass cloth, with a layer of rovings interposed between layers of mat according to one embodiment; an inner liner of high purified asbestos known as Quintex II is further included. U.S. Pat. No. 4,161,714 (Jacobs, Jr. 1979) describes another composite resin-impregnated fuse tube, this one having an outer layer of glass cloth, one or more intermediate layers of mat, and an inner layer of cloth.
Despite these proposals, the fuse tubes known in the art do not adequately meet the various criteria discussed above. There remains a need for fuse tubes and other tubular products which have an improved burst strength while being compact, which can be produced accurately, efficiently, and inexpensively with reduced amounts of downtime caused by product breakage and the like, which have improved dimensional stability, and which perform their intended purposes reliably and for long periods of time.